Conventionally, in order to improve a linear characteristic and a modulation factor of the ASK modulation, a technique by which high frequency signals (or HF signals) are amplified according to levels of modulating signals has been disclosed.
In order to increase the modulation factor, it is important to reduce a modulating output signal that is generated at the time when the level of the input signal is low. On the other hand, in order to output a modulating output signal with a large amplification, it is necessary to set an amplifier's gain high. Due to the incompatibility of reducing the modulating output signal and the need to obtain a modulating output signal that has a large amplification, there is a need for further development, i.e., for a device that can provide a modulating output signal with large amplification while reducing the modulating output signal.